The need for high performance and reliable energy storage in the modern society is well documented. Lithium batteries represent a very attractive solution to these energy needs due to their superior energy density and high performance. However, available Li-ion storage materials limit the specific energy of conventional Li-ion batteries. While lithium has one of the highest specific capacities of any anode (3861 mAh/g), typical cathode materials such as MnO2, V2O5, LiCoO2 and (CF)n have specific capacities less than 200 mAh/g.
Recently, lithium/oxygen (Li/O2) or lithium air batteries have been suggested as a means for avoiding the limitations of today's lithium ion cells. In these batteries, lithium metal anodes are used to maximize anode capacity and the cathode capacity of Li air batteries is maximized by not storing the cathode active material in the battery. Instead, ambient O2 is reduced on a catalytic air electrode to form O22−, where it reacts with Li+ ions conducted from the anode. Aqueous lithium air batteries have been found to suffer from corrosion of the Li anode by water and suffer from less than optimum capacity because of the excess water required for effective operation.
Abraham and Jiang (J. Electrochem. Soc., 1996, 143 (1), 1-5) reported a non-aqueous Li/O2 battery with an open circuit voltage close to 3 V, an operating voltage of 2.0 to 2.8 V, good coulomb efficiency, and some re-chargeability, but with severe capacity fade, limiting the lifetime to only a few cycles. Further, in non-aqueous cells, the electrolyte has to wet the lithium oxygen reaction product in order for it to be electrolyzed during recharge. It has been found that the limited solubility of the reaction product in available organic electrolytes necessitates the use of excess amounts of electrolyte to adequately wet the extremely high surface area nanoscale discharge deposits produced in the cathode. Thus, the required excess electrolyte significantly decreases high energy density that would otherwise be available in lithium oxygen cells.
Operation of Li/O2 cells depends on the diffusion of oxygen into the air cathode. Oxygen absorption is a function of the electrolyte's Bunsen coefficient (α), electrolyte conductivity (σ), and viscosity (η). It is known that as the solvent's viscosity increases, there are decreases in lithium reaction capacity and Bunsen coefficients. Additionally, the electrolyte has an even more direct effect on overall cell capacity as the ability to dissolve reaction product is crucial. This problem has persisted in one form or another in known batteries.
Indeed, high rates of capacity fade remain a problem for non-aqueous rechargeable lithium air batteries and have represented a significant barrier to their commercialization. The high fade is attributed primarily to parasitic reactions occurring between the electrolyte and the mossy lithium powder and dendrites formed at the anode-electrolyte interface during cell recharge, as well as the passivation reactions between the electrolyte and the LiO2 radical which occurs as an intermediate step in reducing Li2O2 during recharge.
During recharge, lithium ions are conducted across the electrolyte separator with lithium being plated at the anode. The recharge process can be complicated by the formation of low density lithium dendrites and lithium powder as opposed to a dense lithium metal film. In addition to passivation reactions with the electrolyte, the mossy lithium formed during recharge can be oxidized in the presence of oxygen into mossy lithium oxide. A thick layer of lithium oxide and/or electrolyte passivation reaction product on the anode can increase the impedance of the cell and thereby lower performance. Formation of mossy lithium with cycling can also result in large amounts of lithium being disconnected within the cell and thereby being rendered ineffective.
Lithium dendrites can penetrate the separator, resulting in internal short circuits within the cell. Repeated cycling causes the electrolyte to break down, in addition to reducing the oxygen passivation material coated on the anode surface. This results in the formation of a layer composed of mossy lithium, lithium-oxide and lithium-electrolyte reaction products at the metal anode's surface which drives up cell impedance and consumes the electrolyte, bringing about cell dry out.
Attempts to use active (non lithium metal) anodes to eliminate dendritic lithium plating have not been successful because of the similarities in the structure of the anode and cathode. In such lithium air “ion” batteries, both the anode and cathode contain carbon or another electronic conductor as a medium for providing electronic continuity. Carbon black in the cathode provides electronic continuity and reaction sites for lithium oxide formation. To form an active anode, graphitic carbon is included in the anode for intercalation of lithium and carbon black is included for electronic continuity. Unfortunately, the use of graphite and carbon black in the anode can also provide reaction sites for lithium oxide formation. At a reaction potential of approximately 3 volts relative to the low voltage of lithium intercalation into graphite, oxygen reactions would dominate in the anode as well as in the cathode. Applying existing lithium ion battery construction techniques to lithium oxygen cells would allow oxygen to diffuse throughout all elements of the cell structure. With lithium/oxygen reactions occurring in both the anode and cathode, creation of a voltage potential differential between the two is difficult. An equal oxidation reaction potential would exist within the two electrodes, resulting in no voltage.
As a solution to the problem of dendritic lithium plating and uncontrolled oxygen diffusion, known aqueous and non-aqueous lithium air batteries have included a barrier electrolyte separator, typically a ceramic material, to protect the lithium anode and provide a hard surface onto which lithium can be plated during recharge. However, formation of a reliable, cost effective barrier has been difficult. A lithium air cell employing a protective solid state lithium ion conductive barrier as a separator to protect lithium in a lithium air cell is described in U.S. Pat. No. 7,691,536 of Johnson. Thin film barriers have limited effectiveness in withstanding the mechanical stress associated with stripping and plating lithium at the anode or the swelling and contraction of the cathode during cycling. Moreover, thick lithium ion conductive ceramic plates, while offering excellent protective barrier properties, are extremely difficult to fabricate, add significant mass to the cell, and are rather expensive to make.
As it relates to the cathode, the dramatic decrease in cell capacity as the discharge rate is increased is attributed to the accumulation of reaction product in the cathode. At high discharge rate, oxygen entering the cathode at its surface does not have an opportunity to diffuse or otherwise transition to reaction sites deeper within the cathode. The discharge reactions occur at the cathode surface, resulting in the formation of a reaction product crust that seals the surface of the cathode and prevents additional oxygen from entering. Starved of oxygen, the discharge process cannot be sustained.
Another significant challenge with lithium air cells has been electrolyte stability within the cathode. The primary discharge product in lithium oxygen cells is Li2O2. During recharge, the resulting lithium oxygen radical, LiO2, an intermediate product which occurs while electrolyzing Li2O2, aggressively attacks and decomposes the electrolyte within the cathode, causing it to lose its effectiveness.
High temperature molten salts have been suggested as an alternative to organic electrolytes in non-aqueous lithium-air cells. U.S. Pat. No. 4,803,134 of Sammells describes a high lithium-oxygen secondary cell in which a ceramic oxygen ion conductor is employed. The cell includes a lithium-containing negative electrode in contact with a lithium ion conducting molten salt electrolyte, LiF—LiCl—Li2O, separated from the positive electrode by the oxygen ion conducting solid electrolyte. The ion conductivity limitations of available solid oxide electrolytes require that such a cell be operated in the 700° C. range or higher in order to have reasonable charge/discharge cycle rates. The geometry of the cell is such that the discharge reaction product accumulates within the molten salt between the anode and the solid oxide electrolyte. The required space is an additional source of impedance within the cell.
TABLE 1Physical properties of Molten Nitrate ElectrolytesMelt Tempκ (S/cm)SystemMol %° C.@570Kat Mol %LiNO3—KNO342-581240.68750.12 mol %LiNO3LiNO3—RbNO330-701480.539  50 mol %RbNO3NaNO3—RbNO344-561780.519  50 mol %RbNO3LiNO3—NaNO356-441870.98549.96 mol %NaNO3NaNO3—KNO346-542220.6650.31 mol %NaNO3KNO3—RbNO330-702900.394  70 mol %RbNO3
Molten nitrates also offer a viable solution and the physical properties of molten nitrate electrolytes are summarized in Table 1 (taken from Lithium Batteries Using Molten Nitrate Electrolytes by Melvin H. Miles; Research Department (Code 4T4220D); Naval Air Warfare Center Weapons Division; China Luke, Calif. 93555-61000).
The electrochemical oxidation of the molten LiNO3 occurs at about 1.1 V vs. Ag+/Ag or 4.5 V vs. Li+/Li. The electrochemical reduction of LiNO3 occurs at about −0.9V vs. Ag+/Ag, and thus these two reactions define a 2.0V electrochemical stability region for molten LiNO3 at 300° C. and are defined as follows:LiNO3→Li++NO2+½O2+e−  (Equation 1)LiNO3+2e−→LiNO2+O−−  (Equation 2)
This work with molten nitrates was not performed with lithium air cells in mind; however, the effective operating voltage window for the electrolyte is suitable for such an application. As indicated by the reaction potential line in FIG. 1, applying a recharge voltage of 4.5V referenced to the lithium anode can cause lithium nitrate to decompose to lithium nitrite, releasing oxygen. On the other hand, lithium can reduce LiNO3 to Li2O and LiNO2. This reaction occurs when the LiNO3 voltage drops below 2.5V relative to lithium. As long as there is dissolved oxygen in the electrolyte, the reaction kinetics will favor the lithium oxygen reactions over LiNO3 reduction. Oxide ions are readily converted to peroxide (O22−) and aggressive superoxide (O2−) ions in NaNO3 and KNO3 melts (M. H. Miles et al., J. Electrochem. Soc., 127,1761 (1980)).
A need remains for a lithium air cell which overcomes problems associated with those of the prior art.